Thermoptic materials exhibit changes in physical properties such as electrical conductivity, absorption, reflectivity and refractive index as the result of state changes between the semiconductor and metal states. Thermoptic materials, such as vanadium and titanium oxides, have relatively low absorption in the semiconductor state and high absorption and high reflectance in the metal state.
Thermoptic materials which exhibit metal-semiconductor phase transitions have a reflectance which is temperature sensitive. In thin films of thermoptic materials, there is a transition temperature below which the film behaves as a semiconductor and above which it behaves as a metal. Above the transition temperature of the thin film, the reflectance of the thermoptic material increases abruptly. It is well known that images can be recorded in thin films of thermoptic material at extremely high speeds by selectively heating a region of the film with a laser or electron beam. As long as the ambient temperature of the film is maintained at a suitable bias temperature inside the hysteresis loop, these images will be stored in the film. Normally, such images are erased by allowing the entire film to cool from its bias temperature down to some temperature well below the hysteresis loop.
This radiative cooling, however, is extremely slow and impractical. More direct means, such as those disclosed in U.S. Pat. No. 4,283,113 to Eden, have also been employed for cooling these films, such as direct cooling with thermoelectric junctions. U.S. Pat. No. 4,236,156 to Eden discloses a method for erasing thermoptic film displays by generating a high frequency surface acoustic wave on a piezeoelectric substrate on which a thermoptic film is employed. Another technique was to erase the film using a short burst spray of liquid refrigerant to cool the film below its hysteresis loop in a few milliseconds. However, the prior art techniques for erasing the rebiasing thermoptic films are neither fast enough nor practical in many applications. For example, in dynamic infrared scene simulation, where a laser or electron beam is used to write scene information on the thermoptic modulator, cycling requirements may require writing and erasing a scene at speeds as high as about 20 milliseconds per cycle. Since prior art erasing techniques are too slow and require complex and expensive apparatus, a faster method and simple apparatus are needed for erasing and rebiasing thermoptic thin film modulators.